Forming pressure-welded joints

ABSTRACT

A method of forming a welded joint between aluminium and steel without the formation of intermetallics at the aluminium/steel interface is achieved by driving a wedge shaped steel member into a heated aluminium slug located in a restraining die. The driving is performed under such conditions that the aluminium/steel interface is maintained substantially free from extraneous matter so that the aluminium and steel become welded to one another (without fusion of aluminium) under the high pressure resulting from the extrusion of the aluminium into the space between the die wall and the shank of the steel member during the driving of the steel member into the slug. The temperature of the slug is preferably in the range of 400* to 550*C and the wedge member is driven quickly (i.e., within 10 seconds) to prevent it from becoming heated and acquiring a surface oxide film.

United States Patent [191 Hirschfield et al.

[451 Dec. 31, 1974 FORMING PRESSURE-WELDED JOINTS [73] Assignee: Alcan Research and Development Limited, Montreal, Quebec, Canada [22] Filed: Dec. 13, 1972 [21] Appl. No.: 314,666

[30] Foreign Application Priority Data Dec. 14, 1971 Great Britain 58111/71 [52] US. Cl 29/482, 29/493, 29/497.5 [51] Int. Cl B23k 31/02 [58] Field of Search 29/482, 470.1, 497.5, 493

[56] References Cited UNITED STATES PATENTS 2,807,082 9/1957 Zambrow et a1 29/497.5 X 3,234,772 2/ 1966 Fehling et a1. 29/470.1 3,436,806 4/1969 Supan 29/482 3,474,523 10/1969 Musso et a1 29/497.5 X 3,634,934 l/l972 Fitzgerald 29/470.l X 3,664,012 5/1972 Wilke et a1. 29/470.1 X

10/1972 Walker 29/497.5 X 11/1972 Batista 29/497.5 X

Primary ExaminerRichard B. Lazarus Attorney, Agent, or FirmCooper, Dunham, Clark, Griffin & Moran 5 7 ABSTRACT A method of forming a welded joint between aluminium and steel without the formation of intermetallics at the aluminium/steel interface is achieved by driving a wedge shaped steel member into a heated aluminium slug located in a restraining die. The driving is performed under such conditions that the aluminium/- steel interface is maintained substantially free from extraneous matter so that the aluminium and steel become welded to one another (without fusion of aluminium) under the high pressure resulting from the extrusion of the aluminium into the space between the die wall and the shank of the steel member during the driving of the steel member into the slug. The temperature of the slug is preferably in the range of 400 to 550C and the wedge member is driven quickly (i.e., within 10 seconds) to prevent it from becoming heated and acquiring a surface oxide film.

11 Claims, 6 Drawing Figures PATENTED 3,857. 163

SHEET 1 [IF 2 1 FORMING PRESSURE-WELDED JOINTS DESCRIPTION The present invention relates to a method of forming a welded joint between aluminium and steel, the joint being suitable for the transmission of a heavy electric current and having a high mechanical strength. A typical application of the method of the invention is to the formation of joints between aluminium and steel in cathode riser connections and anode rod connections for electrolytic reduction cells for the production of aluminium. The method of the present invention is also suitable for the formation of structural joints of high strength between aluminium and steel. Thus the present invention may be employed to form a composite member for connecting an aluminium vessel to a steel support structure.

Methods ofjoining aluminium to steel, which lead to the formation of brittle intermetallic compounds at the interface between the steel and the aluminium, are relatively unsatisfactory both from the standpoint of mechanical strength and electrical conductivity. It has been found in the case of anode rod connections that a proportion fail during early stages of the anode life when produced by methods, such as arc welding or casting aluminium onto steel, which involve fusion of aluminium and consequent growth of intermetallics at the aluminium/steel interface. It is an object of the present invention to provide a method of welding aluminium to steel in a joint by providing sufficient pressure without generating sufficient temperature to lead to the growth of intermetallic compounds at the interface.

In its broadest aspects the present invention provides a method of making a welded joint between aluminium and steel, which comprises placing a heated aluminium slug in a restraining die and pressing a steel member formed into the slug in such a manner that virgin aluminium and clean steel are brought into intimate contact under conditions of high pressure at the aluminium steel interface as the aluminium extrudes outwardly around the shank portion of the steel member and no substantial quanitty of extraneous material is drawn into the aluminium slug by the said member.

The temperature of the aluminium slug is preferably in the range of 400 550C., most preferably in the range of 475 525C, which allows the steel member to penetrate into the aluminium without excessive pressure. However the pressure applied should be sufficient to allow the steel member to be driven into the aluminium rapidly if the aluminium is heated to a temperature of the order above indicated. When a temperature in the above quoted range is employed, the initially unheated steel member (typically 1 inch thick) should be driven in a period of about 3 seconds to avoid excessive oxidation of the steel by reason of take-up of heat from the aluminium. There should be sufficient pressure developed at the aluminium/steel interface to form a bond at a temperature which is not great enough to result in the formation of intermetallics. This pressure is dependent on four parameters: (1) the temperature of the aluminium slug (2) the angles on the wedge, on the shank, and on the die wall (3) the die cavity width (the difference between the die cavity width and the shank thickness) and (4) the friction between the die wall and the extruding aluminium.

The steel has a wedge-shaped edge portion, having an edge with an apex angle of say 30, although somewhat greater or lesser apex angles may be employed, and a shank portion. The end faces of the steel member are also preferably somewhat convergent to assist in tne entry of the steel into the aluminium slug. The shape of the shank of the steel member and the corresponding part of the die side (the part of the die side which lies immediately outwardly of the shank when the shank penetrates the aluminium) are preferably arranged so that there is a small amount of choke between the die side and the shank, so that the aluminium which is caused to flow between the die sides and the shank is extruded into a tapering space. The extrusion of the metal into a tapering space assists in the development of the high pressure at the steel/aluminium interface required to secure pressure welding without fusion of the aluminium. Where the space between the shank of the wedge member and the die wall is small, such as one-fourth inch or less, it is possible to develop sufficient pressure between the aluminium and the steel wedge member, even though the faces of the shank are parallel to the die sides, provided there is sufficient friction between the extruding aluminium and the die wall.

The provision of a taper between the die sides and the shank of the steel member may be achieved by providing parallel opposed faces on the die sides in the relevant area in conjunction with steel members having slightly tapering shanks. However, it is preferred that the shanks of the steel members be parallel-sided and the die sides provided with a small amount of reverse taper, preferably from 1 to 5, most preferably about 3. It is even possible for a small amount of reverse taper to be provided on the shanks, providing that this is less than the reverse taper on the die side. Where the die sides are parallel and the shank of the wedge member is tapered the included angle between the side faces of the shank is preferably in the range of 1 5, most preferably about 3.

Apart from a possible reverse taper towards its mouth, the die cavity is preferably tapered in cross section, although this is not essential to the present invention. The closed end of the die is preferably provided with a longitudinal groove conforming to a suitable edge preparation, for example having an apex angle of for subsequent welding of the aluminium to another aluminium component such as a bus bar.The aluminium slug is shaped to have a cross section which conforms to that of the die cavity and for this purpose is preferably cut from an appropriate extruded section or is in the form of a casting. However the upper end of the slug is preferably formed with a shoulder at each side so that the top /2 1 inch of the slug is held away from contact with the walls of the die during initial penetration of the steel member. In carrying out the process of the invention a lubricant or parting compound between the die wall and the slug is necessary to enable the final welded assembly to be withdrawn from the die and to permit flow of aluminium against the wall of the die. The provision of an initial spacing between the top end of the slug and the die wall avoids excessive drawin of parting lubricant by the steel which would prevent welding wherever it intervened between the aluminium and the steel. Without this precaution there is a tendency for lubricant to be carried up the side wall of the die and across the outer end of the slug and to be drawn into the interior of the slug in contact with the steel, as

a result of the metal flow involved during the pressing operation.

With a wedge member having an apex angle of 30 and a shank of 1 inch thickness the force required to drive the wedge fully into a l -inch thick heated slug within the indicated time interval of 3 seconds is in the range of to 30 tons per inch length of the wedge member. Variations of the apex angle of the wedge within the range of 40 will have relatively little effect on the rate of penetration.

To obtain proper welding of the aluminium to the steel it is extremely important that the steel wedge edge and shank are free from oxides and matter such as grease, oil, etc. To effect this, the stel may be cleaned chemically or mechanically, for example by disc sanding to a surface finish of 60 60 RMS (root mean square). The time interval between cleaning and pressing should be such that the steel remains substantially free from oxides.

It has already been proposed to join aluminium to another higher melting point metal, such as copper by forcing a sharp-edged member formed of the higher melting point metal into an aluminium member, held between a pair of parallel side plates. The weld between the aluminium and the copper was achieved by heating the assembly to cause diffusion to promote the growth of an aluminium-copper eutectic at the interface. Despite the apparent similarity of the two procedures both the objective and the results are very dissimilar since the procedure of the present invention both aims at and results in the pressure welding of steel to aluminium without any detectable layer of intermetallic compounds at the steel-aluminium interface whereas the prior art method aimed at obtaining a eutectic layer at the interface, to act as a hard solder.

Referring now to the accompanying drawings:

FIG. 1 is a cross section of an aluminium slug for the production of an aluminium/steel 18 joint in accordance with the present invention;

FIG. 2 is a cross section of a steel wedge member for use in conjunction with the slug of FIG. 1 and FIG. 3 is a front view of the wedge member of FIG.

FIG. 4 is a diagrammatic cross section of the restrain ing die;

FIG. 5 is a diagrammatic cross section of the completed joint in the die; and

FIG. 6 is a diagrammatic cross section showing an alternative form of slug.

The aluminium slug of FIG. 1 is a casting or cut from an extruded section of an aluminium alloy having suitable physical properties. The slug 1 has sides 2 which taper from top to bottom by about 3 percent. The bottom end has faces 3 which meet at an apex 4 at an angle of about 100. At the upper or outer end of the side faces 2 the width of the slug is reduced by the provision of a pair of shoulders 5, so that the faces 2' are initially maintained out of contact with the sides of the dies.

The wedge member 6 of FIGS. 2 and 3 which may, for example, be fabricated from steel plate, has a main body portion 7 of a shape dictated by the service which the joint is to perform. Thus the main body portion 7 may be shaped for connection to another structure by means of bolts or welding or other suitable means. The wedge member 6 is provided with a shank portion 8 and a wedge-shaped edge portion 9. It is with the side faces of the shank portion 8 and of the wedge-shaped edge portion 9 that a strong pressure-welded bond is formed by the method of the present invention.

The restraining die 10 of FIG. 4, within which the joint between the aluminium alloy slug 1 and the wedge member 6 is formed, comprises an outer ring 11, having a tapered central aperture 12 to receive die side members 14 and a die base member 15, which is provided with an ejector member 16.

The outer surfaces of the side members 14 conform in curvature and taper with the central aperture in ring 11. The inner surfaces of the side members are shaped to provide a die space with sides corresponding to the taper of the sides 2 of the aluminium slug 1, whilst the die base member 15 is longitudinally grooved to conform with the shape of the faces 3 of the slug 1. The opposed faces of the member 14 are provided with inclined portions 17 to define the sides of the tapered space to receive the slug 1 and reverse tapered portions 18 which commence at a level approximately corresponding with the shoulders 5 of the slug 1, with the result that when the wedge member 6is pressed into the slug 1 the upper part of the slug above the shoulders 5 moves laterally to the wedge member 6 whilst the upper part of the slug below the shoulders Sis extruded upwardly into the choke space defined between the reverse tapered face portions 18 and the parallel shank 8. The reverse tapered sides are inclined at an angle of /z 2 /2", preferably about 1 /2 to the medial plane of the die cavity, so that the included angle between the faces 18 is 1 5, preferably about 3.

To form the joint the slug is first heated to a temperature in the range of 475 525C, after which it is placed into the die. The freshly cleaned wedge-shaped steel member 6 is then placed in position and pressed downwardly into the die by a hydraulic ram, which moves at a speed sufficient to drive the wedge to the position illustrated in FIG. 5 in a time interval of 3 10 seconds. The assembly of slug 1 and wedge member 6 is then ejected from the die by pressing the bottom member 15 upwardly, which lifts the side members 14 in relation to the ring 11 and permits them to separate from each other and from the completed joint.

The aluminium/steel pressure-welded joint assembly may then be connected to aluminium and to steel members respectively by appropriate welding techniques.

In tests performed on joint assemblies produced by this method it was found that, by reason of the absence of intermetallics, the aluminium failed in tension without failure of the pressure bond at the aluminium/steel interface. Furthermore the electrical resistance of the aluminium/steel interface was found to be very satisfactory.

As an alternative to the use of the aluminium slug shown in FIG. 1, a slug 19 may be cut from plate, as shown in FIG. 6. The final shape is obtained during the pressing operation in which the aluminium is forged to the shape of the die cavity. The plate slug 19 is cen' tered in the die cavity by means of two narrow aluminium spacers 20. The spacers prevent contact of the plate slug 19 with the parting lubricant on the walls of the die cavity and thus perform the same function as the shoulders 5 on the extruded slug 1. As will be seen, the spacers are shaped so that they may be put in position before the heated slug is introduced into the die cavity. For this purpose they are provided with flanges which rest on the top of the die.

In this arrangement the initial penetration of the wedge-shaped portion 9 presses the plate slug 19 downward to conform with the die cavity, while the top portion (about one-half inch) of the plate moves towards the wedge to bring its sides into contact with the reverse tapered portions 18 of the die cavity. The combination of this expansion and the downward pressure exerted on the heated slug results in the deformation of the plate slug to bring it into conformity with the walls of the die space. When the whole of the die space has thus been filled with metal, further driving of the wedge member results in upward flow of aluminium into the choke space between the reverse tapered faces 18 and the shank 8 of the wedge member 6 and this results in the high pressure at the steel/aluminium interface required for the pressure welding of aluminium to steel.

In another example the opposed faces of the die are parallel throughout their length. The slug is made from heavy gauge aluminium plate, as in the case of FIG. 6. In this instance the shank of the wedge member is tapered to define, in conjunction with the side walls of the die, the choke space into which the extruded metal flows in the course of the formation of the pressure weld.

The plate may be slightly thinner than the width of the die and initially held in position by means of appropriately spaced spacers as in the arrangement of FIG. 6 so as to maintain an initial absence of contact between the slug and the lubricant on the die walls. Alternatively the two side faces of the slug may be machined to provide recessed surfaces, similar to those shown at 2' in FIG. 1.

We claim:

1. A method of'forming a welded joint between aluminium and steel without formation of significant quantities of intermetallic phases at the steelaluminium interface which comprises driving a straight-edged steel wedge member into one end of a heated aluminium member located in a restraining die having side walls and a closed end remote from said one end of said aluminium member, the die and aluminium member being so mutually shaped that a clearance is provided initially between the aluminium and the side wall of the die adjacent said one end of the alu- 4 minium member to avoid the introduction of extraneous material from the side surface of the aluminium to the aluminium/steel interface, said wedge member having bare steel surfaces for direct contact with aluminium and further having a wedge-shaped edge portion and a shank portion, while maintaining the shank portion spaced from the die walls, and extruding aluminium of said aluminium member into the space between the die walls and the shank during the driving of the wedge member under conditions arranged to develop at the steel-aluminium interface, a pressure sufficient to cause pressure welding of aluminium to steel without fusion of the aluminium.

2. A method according to claim 1 including heating the aluminium member to 400 550C before driving the wedge member, and completing the driving of the wedge member in a period of 3 seconds to avoid undue heating of the steel member before completion of the formation of the joint.

3. A method according to claim 2 in which the extruding step comprises extruding the aluminium between a shank portion and corresponding portions of the die walls shaped to define a choke zone tapering in a direction away from said closed die end.

4. A method according to claim 3 wherein the extruding step comprises extruding the aluminium into spaces respectively defined between opposed side faces of the shank portion which are substantially parallel to each other and corresponding portions of the die walls which are inclined to each other at an included angle in the range of 1 5.

5. A method according to claim 1 including providing initial clearance between said aluminium member and the die walls adjacent said one end of the aluminium member by disposing in said die an aluminium member having an initial cross-sectional shape which corresponds to that of the corresponding portion of the cavity defined by the die, except that a recessed shoulder is provided at each side of the aluminium member adjacent said one end thereof.

6. A method according to claim 1 including disposing in the die an aluminium member having an initial crosssectional width less than that of the cavity defined by the die, and placing, in the end of the die remote from said closed end, aluminium spacers extending downwardly for a sufficient distance to maintain the portion of the aluminium member adjacent said one end thereof out of contact with the walls of the die.

7. A method according to claim 1 including disposing said aluminium member in a cavity, defined by the side walls of a die having a closed lower end, which has a downwardly tapering lower portion and an upwardly tapering upper portion to provide a choke zone in conjunction with the shank of the wedge member.

8. A method according to claim 1, including the step of cleaning the surfaces of said wedge member before performing the driving step, for assured exposure of bare steel of said wedge member for direct contact with aluminium of said aluminium member.

9. A method of producing a direct pressure-welded joint between aluminium and steel free of significant quantities of intermetallic phases at the steelaluminium interface, comprising a. disposing a heated aluminium slug in a die cavity defined by opposed side walls and a closed lower end of an upwardly opening die, with one end of said slug facing upwardly, while b. initially spacing at least the upper end portion of said slug from said die walls, and

c. driving a straight-edged steel wedge member downwardly into the upper end of the heated slug in the die for causing pressure-welding of the aluminium of the slug to the steel of the wedge member without fusion of the aluminium, said wedge member having bare steel surfaces for direct steelaluminium contact, said wedge member further having a leading wedge-shaped edge portion and a following shank portion with opposed sides respectively facing the opposed side walls of the die during the driving step, while d. maintaining the sides of said shank portion spaced from the respectively facing side walls of the die for defining upwardly opening spaces therebetween, during the driving step, and

e. extruding aluminium of the slug upwardly into said upwardly opening spaces during and incident to said driving step.

10. A method according to claim 9, wherein the extruding step comprises extruding the aluminium into 1 l. A method according to claim 9, including heating the slug, for the driving step, to a temperature between about 400 and about 550C, and performing the driving step in a total period of about 3 to about l sectaper angle, in each of said spaces, of between about /2 5 onds.

and about 2 /z. 

1. A method of forming a welded joint between aluminium and steel without formation of significant quantities of intermetallic phases at the steel-aluminium interface which comprises driving a straight-edged steel wedge member into one end of a heated aluminium member located in a restraining die having side walls and a closed end remote from said one end of said aluminium member, the die and aluminium member being so mutually shaped that a clearance is provided initially between the aluminium and the side wall of the die adjacent said one end of the aluminium member to avoid the introduction of extraneous material from the side surface of the aluminium to the aluminium/steel interface, said wedge member having bare steel surfaces for direct contact with aluminium and further having a wedge-shaped edge portion and a shank portion, while maintaining the shank portion spaced from the die walls, and extruding aluminium of said aluminium member into the space between the die walls and the shank during the driving of the wedge member under conditions arranged to develop at the steel-aluminium interface, a pressure sufficient to cause pressure welding of aluminium to steel without fusion of the aluminium.
 2. A method according to claim 1 including heating the aluminium member to 400* - 550*C before driving the wedge member, and completing the driving of the wedge member in a period of 3 - 10 seconds to avoid undue heating of the steel member before completion of the formation of the joint.
 3. A method according to claim 2 in which the extruding step comprises extruding the aluminium between a shank portion and corresponding portions of the die walls shaped to define a choke zone tapering in a direction away from said closed die end.
 4. A method according to claim 3 wherein the extruding step comprises extruding the aluminium into spaces respectively defined between opposed side faces of the shank portion which are substantially parallel to each other and corresponding portions of the die walls which are inclined to each other at an included angle in the range of 1* - 5*.
 5. A method according to claim 1 including providing initial clearance between said aluminium member and the die walls adjacent said one end of the aluminium member by disposing in said die an aluminium member having an initial cross-sectional shape which corresponds to that of the corresponding portion of the cavity defined by the die, except that a recessed shoulder is provided at each side of the aluminium member adjacent said one end thereof.
 6. A method according to claim 1 including disposing in the die an aluminium member having an initial cross-sectional width less than that of the cavity defined by the die, and placing, in the end of the die remote from said closed end, aluminium spacers extending downwardly for a sufficient distance to maintain the portion of the aluminium member adjacent said one end thereof out of contact with the walls of the die.
 7. A method according to claim 1 including disposing said aluminium member in a cavity, defined by the side walls of a die having a closed lower end, which has a downwardly tapering lower portion and an upwardly tapering upper portion to provide a choke zone in conjunction with the shank of the wedge member.
 8. A method according to claim 1, including the step of cleaning the surfaces of said wedge member before performing the driving step, for assured exposure of bare steel of said wedge member for direct contact with aluminium of said aluminium member.
 9. A method of producing a direct pressure-welded joint between aluminium and steel free of significant quantities of intermetallic phases at the steel-aluminium interface, comprising a. disposing a heated aluminium slug in a die cavity defined by opposed side walls and a closed lower end of an upwardly opening die, with one end of said slug facing upwardly, while b. initially spacing at least the upper end portion of said slug from said die walls, and c. driving a straight-edged steel wedge member downwardly into the upper end of the heated slug in the die for causing pressure-welding of the aluminium of the slug to the steel of the wedge member without fusion of the aluminium, said wedge member having bare steel surfaces for direct steel-aluminium contact, said wedge member further having a leading wedge-shaped edge portion and a following shank portion with opposed sides respectively facing the opposed side walls of the die during the driving step, while d. maintaining the sides of said shank portion spaced from the respectively facing side walls of the die for defining upwardly opening spaces therebetween, during the driving step, and e. extruding aluminium of the slug upwardly into said upwardly opening spaces during and incident to said driving step.
 10. A method according to claim 9, wherein the extruding step comprises extruding the aluminium into upwardly tapering and upwardly opening spaces respectively defined between side portions of the shank and respectively facing side portions of the die wall cooperatively oriented to provide an included upward taper angle, in each of said spaces, of between about 1/2 * and about 2 1/2 *.
 11. A method according to claim 9, including heating the slug, for the driving step, to a temperature between about 400* and about 550*C, and performing the driving step in a total period of about 3 to about 10 seconds. 